A win will ensure the Mystics will qualify for post-season play for the first time, but anything less will see the talent-laden side fail to join the Waikato-Bay of Plenty Magic as New Zealand's representatives in the top four.
The Mystics thumped the Steel by 17 goals in the first round but Fuller is only too aware of the coaching nous of Robyn Broughton and the ability of the Southerners to cause an upset.
"You can't go into this game underestimating anything about the Steel, and you never do, because of the way Robyn prepares her team.
"It's another game we've got to win. The ball is in our court - for want of a better word - but we've really got to go down there and play positive and attacking netball.
"I think it is good to have pressure like this, it's a must-win game for us and that's the way we're going to approach it," said Fuller.
She said nothing could be read into their big win early in the season as the Steel were robbed of key players Natasha Chokljat, Sheryl Scanlan and Daneka Wipiiti for that match and have shown in recent weeks signs they were getting back to top form.
The Mystics have paid special attention at training on how to break down the Steel's zonal defence and how to cope with their style of possession netball.
Fuller will be able to call on a fit-again Joline Henry as the defender continues to make her comeback from a troublesome ankle injury and she saw her return to fitness as vital at this stage of the season.
"It's good to have a player of her calibre and her experience at full strength - without injury - going into these last games," she said.
Cathrine Latu will be motivated to put in a good performance after the shooter's bid to play for the Silver Ferns in the world championships was scuppered by a ruling by the Court of Arbitration for Sport this week but Fuller said she was not dwelling on the decision.
"She's dumped it, we've dumped it and we're moving on," she said.
In other matches, the Canterbury Tactix host the unbeaten Queensland Firebirds on Saturday night, while third placed Melbourne Vixens entertain the fourth placed NSW Swifts with the winner banking their place in the playoffs and the loser at risk of dropping out of the top four if the Mystics win.
On Monday, the Adelaide Thunderbirds face the West Coast Fever with both out of contention, while the Magic can seal their second place by beating the Central Pulse in Taupo.
